Amplifier Version 2.0

By
Chad Rempp
2008/06/16

Version 2 of my amplifier is a Class-AB amp using the LM3876 and delivers 56
watts continuous and 100 watts peak output. None of these circuits are my own
design, everything come from a great website,
Elliott Sound Products This is only an
overview of my construction of their design. There are many very informative
articles at ESP that explain much more and much better than what I provide here.
I really suggest looking through the site.

The construction will consist of three parts:

Power Supply

Pre-Amp

Power-Amp

Power Supply

The power supply will take 120V AC and step it down to 25V and then rectify it
to ±15V for the pre-amp and ±35V for the power-amp.

FIG 1. Power Supply Circuit

The power supply will probably be the most expensive part of the amplifier.
Most of the cost is for a clean, high power transformer.

Part

Description

Source

Part #

Quantity

Each

Cost

R1

10Ω 5W wirewound

Jameco

660375

1

$0.36

$0.36

R2-3

2.2KΩ 1W 1%

Mouser

71-RS1A-2.2K

2

$1.15

$2.30

R4

10kΩ 1W

On-Hand

N/A

1

N/A

N/A

R5-6

680Ω 1W 1%

Mouser

71-RS1A-680

2

$1.26

$2.52

C1

100nF 50V (.1μF)

Jameco

609545

1

$0.22

$0.22

C2-5

10,000μF 50V

Jameco

157738

4

$4.95

$19.80

C6-7

100μF 50V

Jameco

29964

2

$0.08

$0.16

D1-2

5A Diode

Mouser

640-6FM120

2

$0.86

$1.72

D3-4

Zener 15V

Jameco

804818

2

$0.19

$0.38

BR1

Bridge Rectifier 35A (chassis mount)

Jameco

178503

1

$3.25

$3.25

F1

3A Slo-Blo Fuse

Jameco

69535

1

$0.49

$0.49

SW1

STDP switch

On-hand

N/A

1

N/A

N/A

T1

Toroidal Transformer 25-0-25

AnTek

AN-2225

1

$29.00

$29.00

Total

$60.20

Construction

Perhaps the most important component of the power supply is the transformer.
After a week of research and scouring the web I found a manufacturer of good
quality toroidal transformers that were half the price of any other place I
looked at.

FIG 2. My Transformer!!!

It took me a while to figure out how to wire the transformer for a single coil
on the primary and a center-tap on the secondary. I finally just guessed and
stood far away when I tested it.
Here's the data sheet for my transformer. On the primary connect the
center black and red leads to form one long coil. On the secondary connect
the center blue and green leads together form a center tap. On the transform
I have the leads are come out of the wrapper in the order listed on the data
sheet.

FIG 3. Transformer mounted and wired

I haven't decided on what to use for a case yet so I am mounting everything in
old computer PSU enclosures. The first box is the mains wiring and the
transformer with AC 25 Volt, ground, and earth coming out.

FIG 4. Transformer and mains enclosure

FIG 5. Transformer and mains enclosure

After adding the bridge rectifier you can see that the voltage reading is
pretty close to the rated ± 25V of the transformer.

FIG 6. Positive voltage off the bridge rectifier

FIG 7. Negative voltage off the bridge rectifier

Next I needed to build the capacitor bank. It was suggested that the ground of
this section be able to carry large amounts of voltage, something like a copper
bar. I used a small chunk of copper pipe that I cut along the length and
pounded flat. After a broken drill bit and a bit of cutting I had a very nice
ground plate.

FIG 8. Drilling the ground plate

FIG 9. Finished ground plate

Soldering this proved to be a problem. the soldering iron on max couldn't come
close to heating the copper enough. So out comes the blow torch.

FIG 10. Blow torch!!!

After some trial and error and almost melting the caps I finally got it.

FIG 11. Assembled capacitor bank

And now we have filtered ±35V.

FIG 12. Powersupply almost finished

For the ±15V components I decided to make a board using my new circuit board etcher.

FIG 13. Board etcher

I cut a small piece of board, marked where the holes should be, sketched the
traces and drilled the holes.

FIG 14. Preparing the circuit board

According to the instructions that came with my etcher all I needed to do was
draw the traces with an etch resistant pen (Sharpee) and put it in the etcher
for about 10 minutes. This is the result.s

FIG 15. Etching

FIG 16. FAIL!

I tried it again making the traces darker and thicker. That worked a little
better but the board still came out useless. Finally I just ground pits in the
copper to seperate the traces. This worked fine for the power supply but the
large chunks of copper on the board cause capacitance which can disturb the
characteristics of the amplifier design so I'll need to find a different etch
resist.

FIG 17. Improvised board

Overall it turned out OK.

FIG 18. Improvised board put together, top

Soldering was difficult because the smallest drill bit I had was about twice as
big as needed and I put the diodes in backwards.

FIG 19. Improvised board put together, bottom

Now for the test.

FIG 20. +15 Volts

FIG 21. -15 Volts

And everything put together into a nice package. This setup is temporary until
I can find a case for the entire amp.

FIG 22. Power supply finished

FIG 23. Power supply finished

Pre Amplifier

FIG 24. Pre Amplifier Circuit

Part

Description

Source

Part #

Quantity

Each

Cost

R1

100kΩ 1%

Jameco

1867642

2

$0.02

$0.04

R2

6.8kΩ 1%

Mouser

660-MF1/4DCT52R6801F

2

$0.04

$0.08

R3

8.2kΩ 1%

Mouser

660-MF1/4DC8201F

2

$0.03

$0.06

R4-5

10kΩ 1%

Jameco

1867634

4

$0.02

$0.08

R6

100Ω 1%

2

R7

100kΩ

Jameco

1867642

2

$0.02

$0.4

R8

10KΩ Pot

Axe-Man

2

C1

3.3μF 50V

Jameco

607961

2

$0.12

$0.24

IC1

OPA2134 Amp

Jameco

275830

2

$2.29

$4.58

SW1

4 Position DIP

2

SW2

Dual pole 4 position selector

1

Total

$60.20

Construction

The voltage for the preamp and power amp are too high to use a breadboard so I
jumped right in and made boards

FIG 25. Preamp board painted

For the second time (first time was during the construction of amp v1.0) I
forgot that when designing the board layout you must reverse the pins for chips
since you're designing the bottom of the board and the chip goes on top.

FIG 26. Preamp board top (no chip visable)

FIG 27. Preamp board bottom (oops)

FIG 28. Preamp board testing

Power Amplifier

FIG 29. Power Amplifier Circuit

Part

Description

Source

Part #

Quantity

Each

Cost

R1

22kΩ 1%

Mouser

660-MF1/4DC2202F

2

$0.03

$0.06

R2

1kΩ 1%

Jameco

1867626

2

$0.02

$0.04

R3-4

22kΩ 1%

Mouser

660-MF1/4DC2202F

4

$0.03

$0.12

R5

1kΩ 1%

Jameco

1867626

2

$0.02

$0.04

R6

10Ω 1%

2

R7

2.7Ω

2

C1

3.3μF 50V

Jameco

607961

2

$0.12

$0.24

C2-4

100nF 50V (0.1μF)

Jameco

609545

6

$0.22

$1.32

C5-6

22μF 50V

Jameco

93739

4

$0.05

$0.20

C7-8

220μF 50V

Jameco

93780

4

$0.26

$1.04

L1

10 turns of enamel wire wrapped around R6

N/A

N/A

2

N/A

N/A

IC1

LM3876 Opamp

Jameco

126834

2

$5.85

$11.70

F1-2

2 amp, 250 volt Fuse

Jameco

10382

4

$0.24

$0.96

Total

$60.20

Construction

FIG 28. Power Amp top

FIG 28. Power Amp bottom

This is everything connected right before I test it.

FIG 28. Everything connected

After powering on everything worked (it's LOUD) but there is a lot of crackle.
This is likely due to the twist connectors or cold solders. There is also more
hiss than there should be at low volumes. I want to figure out a better way to
bring the grounds back to the caps, this might fix the hiss. And I also need to
find a case.